This invention relates to improved low pressure polymerization processes for preparing linear .alpha.-olefin polymers having ultra high molecular weight.
The low pressure polymerization of .alpha.-olefins with catalyst systems composed of a partially reduced, heavy transition metal component and an organo-metallic reducing component to form high density, solid, linear polymers is well known. Characteristically, the .alpha.-olefin is contacted under relatively mild conditions of pressure and temperarture in a inert hydrocarbon diluent.
In such polymerization process, it is very important to maintain constant reaction conditions, such as temperature and pressure, in order to obtain a high percentage conversion of monomer to polymer. Since these polymerization processes are exothermic in nature, it is generally desirable to provide some means for transfer of bulk heat generated in the polymerization vessel. This is very frequently done by using an indirect heat exchange means in combination with the reactor, for example, by providing a jacket around the reaction vessel through which a suitable coolant is circulated. While this type of heat exchange means does provide adequate heat transfer on a bulk basis, additional control means is required to effect necessary heat transfer on macromolecular basis. Failure to provide such macromolecular heat transfer means results in the softening of the polymer particles being formed. The soft polymer particles then attach to the cooling surfaces of the reactor and cause fouling. Once fouling occurs, the overall bulk temperature of the diluent/polymer slurry cannot be easily controlled thus causing premature shutdown of the reactor.
It has been observed that the problems resulting from inadequate macromolecular heat transfer do not arise when hydrogen is introduced into the reactor in substantial amounts, i.e., generally greater than 40 mole percent based on ethylene. It is found, however, that the introduction of such amounts of hydrogen into the reactor inhibits the formation of ultra high molecular weight polymers which are desirable for many end uses. For example, British Patent No. 991,397 discloses the introduction of hydrogen or some other noncondensible gas into the reactor in sufficient quantity such that the noncondensible gas operates as a direct heat exchanger in the polymerization zone. As noted above, when hydrogen is employed as the non-condensible gas, the molecular weight of the resultant polymer is lowered significantly. On the other hand, when such noncondensible gases are employed in quantities sufficient to effect heat transfer, it is found that the reaction rate is decreased to the point that the reaction generally ceases within a short period of time.
For these reasons, it would be highly desirable to provide a continuous polymerization process for producing ultra high molecular weight linear .alpha.-olefin polymers wherein the process can be maintained at a steady state for a substantial period of time without reactor fouling.